1. Field Of The Invention
This invention relates to a rotary fluid displacement apparatus and more particularly, to an improvement in a rotation preventing and thrust bearing device for an orbiting member fluid displacement apparatus.
2. Description Of The Prior Art
There are several types of fluid apparatus which utilize an orbiting piston or fluid displacing member, such as the scroll-type fluid displacement apparatus disclosed in U.S. Pat. No. 801,182 to Creux.
The scroll-type fluid displacement apparatus disclosed in this U.S. patent includes two scrolls each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset, so that both spiral elements interfit to create a plurality of line contacts between their spiral curved surfaces and thereby to seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces, and as a result, the volume of the fluid pockets changes. Because the volume of the fluid pockets increases or decreases dependent on the direction of the orbiting motion, the scroll-type fluid displacement apparatus is capable of compressing, expanding, or pumping fluids.
Generally, in conventional scroll-type fluid displacement apparatus, one scroll is fixed to a housing and the other scroll, which is the orbiting scroll, is eccentrically supported on a drive (or crank) pin of a rotating drive shaft to produce orbital motion. Such a scroll-type fluid displacement apparatus also includes a rotation preventing device which prevents the rotation of the orbiting scroll and thereby maintains both scrolls in a predetermined angular relationship during operation of the apparatus.
Sealing along the line contacts of such conventional scroll-type apparatus must be maintained because the fluid pockets are defined by the line contacts between the two spiral elements. As the line contacts shift along the surfaces of the spiral elements, the volume of the fluid pockets changes due to the orbital motion of the orbiting scroll. Because the orbiting scroll in such conventional scroll-type apparatus is supported in a cantilever manner, an axial tilt of the orbiting scroll also occurs. Axial tilt occurs because the movement of the orbiting scroll is not rotary motion around the center of the orbiting scroll, but is orbital motion produced by eccentric movement of the drive pin driven by the rotation of the drive shaft. Several problems result from this axial tilt, such as loss of line contact seal, vibration of the apparatus during operation, and noise caused by collisions between the spiral elements.
One simple and direct solution to these problems is the use of a thrust bearing device for carrying the axial thrust load. Thus, scroll-type fluid displacement apparatus have been provided with rotation preventing and thrust bearing devices within their housings.
One recent attempt to improve rotation preventing and thrust bearing devices for scroll-type fluid displacement apparatus is described in U.S. Pat. Nos. 4,160,629 and 4,259,043 to Hidden et al. The rotation preventing and thrust bearing devices in these U.S. patents are integral with one another. The rotation preventing and thrust bearing device described in these U.S. patents (see, e.g., U.S. Pat. No. 4,259,043 (FIG. 7)) comprises one set of indentations formed on the end surface of the circular plate of the orbiting scroll and a second set of indentations formed on an end surface of the fixed plate attached to the housing. A plurality of spheres are placed between facing indentations. Nevertheless, the indentations are formed directly on the end surface of the circular plate of the orbiting scroll or the fixed plate. The production of this type of mechanism, therefore, is very intricate.
Referring to FIGS. 1, 2, and 3, one solution to this disadvantage is described. FIG. 1 is an enlarged, cross-sectional view of a portion of a scroll-type apparatus, and FIG. 2 is an exploded perspective view of the rotation preventing and thrust bearing device 37' of FIG. 1. Rotation preventing and thrust bearing device 37' surrounds boss 273 of orbiting scroll 27. Annular steps 274 and 275, which concentrically surround boss 273, are formed at the end surface of circular end plate 271 opposite to spiral element 272. Annular step 274 is larger radially and closer to spiral element 272; annular step 275 is smaller radially and farther from spiral element 272. Similarly, annular step 113 is formed at the end surface of annular projection 112 of from end plate 11, which rotatably supports a drive shaft (not shown) and is fixedly attached to an opening portion of cup-shaped casing 12. Annular step 113 is concentric with annular projection 112.
Rotation preventing and thrust bearing device 37' include an orbital portion, a fixed portion, and bearings, such as a plurality of balls or spheres. The fixed portion includes (1) first annular race 371 which surrounds annular step 113 in a manner discussed below and (2) first ring 372 fitted against the axial end surface of annular projection 112 of front end plate 11 to overlap the end surface of first annular race 371. First annular race 371 is loosely fitted within annular step 113 because the outer diameter of first annular race 371 is designed to be slightly smaller than the diameter of an annular side wall 113a of annular step 113. First ring 372 is fixedly attached to the axial end surface of annular projection 112 by pins 373. The height of annular side wall 113a of annular step 113 is designed to be greater than the thickness of first annular race 371. The difference between the height of annular side wall 113a of annular step 113 and the thickness of first annular race 371 defines a clearance G between first annular race 371 and first ring 372.
The orbital portion includes (1) second annular race 374 which is disposed within annular step 274 in a manner discussed below and (2) second ring 375 fitted against the axial end surface of annular step 275 to overlap the axial end surface of second annular race 374. Second annular race 374 is loosely fitted within annular step 274 because the inner diameter of second annular race 374 is designed to be slightly greater than the diameter of an annular side wall 274a of annular step 274. Second ring 375 is fixedly attached to the axial end surface of annular step 275 by pins 376. Preferably, the height of annular side wall 274a of annular step 274 is greater than the thickness of second annular race 374. The difference between the height of annular side wall 274a of annular step 274 and the thickness of second annular race 374 also defines a clearance G between second annular race 374 and second ring 375 which is identical to the clearance between the first annular race 371 and the first ring 372.
First ring 372 and second ring 375 each have a plurality of pockets (or holes) 372a and 375a in the axial direction, and the number of pockets in each ring 372 and 375 is equal. Pockets 372a of first ring 372 correspond to or are mirror images of pockets 375a of the second ring 375, i.e., each pair of pockets face each other and have substantially the same size and curvature. Further, the radial distance of the pockets from the center of their respective rings 372 and 375 is the same, i.e., the centers of the pockets are located the same distance from the centers of the rings 372 and 375, respectively. Bearings, such as balls 377, are placed between facing, e.g., substantially aligned, pairs of pockets 372a and 375a.
Referring to FIG. 3, the operation of the rotation preventing and thrust bearing device 37' will be described. In FIG. 3, the center of second ring 375 is located off-center on the right side and the drive shaft rotates in a clockwise direction, as indicated by arrow A. When orbiting scroll 27 is driven by the rotation of the drive shaft, the center of second ring 375 orbits about a circle of radius R.sub.o (together with orbiting scroll 27). Nevertheless, a rotating force, i.e., a moment, which is produced by the offset of the acting point of the reaction force of compression and the acting point of the drive force, acts on orbiting scroll 27. This reaction force tends to rotate orbiting scroll 27 in a clockwise direction about the center of second ring 375. As depicted in FIG. 3, however, eighteen balls 377 may be placed between the corresponding pockets 372a and 375a of rings 372 and 375. In FIG. 3, the interaction between nine balls 377 at the top of the rotation preventing and thrust bearing device and the edges of the pockets 372a and 375a prevents the rotation of orbiting scroll 27. The magnitude of the rotation preventing forces are shown as f.sub.c1-c5 in FIG. 3. As a result of the orbital motion of orbiting scroll 27, the interaction between the nine balls 377 and the edges of the pockets 372a and 375a successively shifts in the direction of the rotation of the drive shaft.
Not only does the reaction force of compression tend to rotate orbiting scroll 27 in the clockwise direction, but it tends to move orbiting scroll 27 forward, i.e., to the left in FIG. 1, and thereby to produce an axial thrust load on an inner end of the drive shaft which is applied through bushing 34. This axial thrust load is carried by the from end plate 11 through second annular race 374, all eighteen balls 377, and first annular race 371. Therefore, each of the eighteen balls 377 comes in contact with the end surfaces of both first and second annular races 371 and 374, and rolls thereon within the corresponding pockets 372a and 375a during the orbital motion of orbiting scroll 27. As balls 377 roll on the axial end surface of first annular race 371, the first annular race 371 freely rotates on the axial end surface of the annular step 113 because of a frictional contact between balls 377 and race 371. As a result, the circular trace of balls 377 on the axial end surface of first annular race 371 is sufficiently dispersed, so that exfoliation of the axial end surface of first annular race 371 should be effectively prevented. Similarly, the second annular race 374 freely rotates on the axial end surface of annular step 274 in the same direction, so that a similar reduction in exfoliation should be achieved.
In the configuration described above, rotation preventing and thrust bearing device 37' consists of a pair of races and a pair of rings, with each race and ring formed separately. Therefore, the parts of rotation preventing and thrust bearing device 37' are easy to construct, and the most suitable material for each part may be individually selected. Generally, in order to be able to bear the axial thrust load and the interacting stress adequately, balls 377, first and second rings 372 and 375, and first and second annular races 371 and 374 are made of stiff and hard material, for example, steel. In order to reduce the weight of the apparatus, however, front end plate 11, casing 12, and the two scroll members may be made of lightweight and relatively soft material, for example, aluminum alloy.
Accordingly, as first annular race 371 freely rotates on the axial end surface of the annular step 113 of front end plate 11 during operation of the apparatus, the axial end surface of first annular race 371 and the axial end surface of annular step 113 come into frictional contact. This frictional contact causes an abnormal abrasion at the softer axial end surface of annular step 113. Therefore, the clearance G between first annular race 371 and first ring 372 becomes greater than that allowable after a short time period during operation of the apparatus, and a similar unacceptable increase occurs in the clearance G between the second annular race 374 and second ring 375. As a result, the apparatus begins to defectively operate after a short time period.